Ink jet printhead having integral filter

1. Field of the Invention

This invention relates to drop-on-demand ink jet printheads and more particularly, to a thermal ink jet printhead having an integral membrane filter over its ink inlet and process for fabricating the printhead with such filter.

2. Description of the Prior Art

A typical thermally actuated drop-on-demand ink jet printing system uses thermal energy pulses to produce vapor bubbles in an ink-filled channel that expels droplets from the channel orifices of the printing system's printhead. Such printheads have one or more ink-filled channels communicating at one end with a relatively small ink supply chamber and having an orifice at the opposite end, also referred to as the nozzle. A thermal energy generator, usually a resistor, is located in the channels near the nozzle at a predetermined distance upstream therefrom. The resistors are individually addressed with a current pulse to momentarily vaporize the ink and form a bubble which expels an ink droplet. A meniscus is formed at each nozzle under a slight negative pressure to prevent ink from weeping therefrom.

U.S. Pat. No. 4,589,952 to Behringer et al discloses a method of making trenches having substantially vertical sidewalls in a silicon substrate using a three-level mask comprising a thick photoresist layer, a silicon nitrite layer, and a thin photoresist layer. Openings are formed in the thin photoresist layer and silicon nitrite layer by reactive ion etching in CF 4 . The openings are continued through the thick photoresist by etching in an atmosphere containing oxygen. The exposed surface of the silicon substrate is then etched in a CF 4 atmosphere containing a low concentration of fluorine. Also disclosed is a method of making an electron beam transmissive mask wherein the openings are made using a three level mask and reactive ion etching of silicon using the etching technique of this invention.

U.S. Pat. No. 4,419,182 to Westerberg et al discloses a method of fabricating screen lenses and aperture array plates wherein a round, flat polish 100 silicon wafer is oxidized and exposed to a pattern photolithographically. The silicon wafer is then indiffused with boron and isotropically etched to produce the predetermined pattern of holes comprising a filter.

U.S. Pat. No. 4,561,789 to Saito discloses a thermal ink transfer printing system containing a thermal printing head comprised of a porous glass substrate which is covered with a polyimide thin film and consequently photoetched to produce a pattern of holes comprising an ink filter.

U.S. Pat. No. 4,417,946 to Bohlen et al discloses a mask for structuring surface areas and a method of manufacture of such mask. The mask includes at least one metal layer with apertures which define the mask pattern and a semiconductive substrate for carrying the metal layer. A semiconductor substrate has through holes that correspond to the mask pattern. The through holes in the semiconductor substrate extend from the metal covered surface on the front to at least one tub shaped recess which extends from the other back surface into the semiconductor substrate. Holes are provided in a surface layer in the semiconductor substrate. The surface layer differs in its doping from the rest of the substrate and the holes which are provided in the surface layer have lateral dimensions larger than the apertures in the metal layer so that the metal layer protrudes over the surface layer.

U.S. Pat. No. 4,639,748 to Drake et al discloses an ink jet printhead having an internal filtering system and fabricating process therefore. Each printhead is composed of two parts aligned and bonded together. One part contains a linear array of heating elements and addressing electrodes on one surface. The other part has a parallel array of elongated recesses for use as ink channels and a common ink supplying manifold recess in communication with the ink channels. The manifold recess contains an integral closed wall defining a chamber with an ink-fill hole. Small passageways are formed in the internal chamber walls to permit passage of ink therefrom into the manifold. Each of the passageways have smaller cross-sectional flow areas than the nozzles to filter the ink, while the total cross-sectional flow area of the passageways is larger than the total cross-sectional flow areas of the nozzles.

U.S. Pat. No. 4,864,329 to Kneezel et al discloses a thermal ink jet printhead having a flat filter placed over the inlet thereof by a fabrication process which laminates a wafer size filter to the aligned and bonded wafers containing a plurality of printheads. The individual printheads are obtained by a sectioning operation, which cuts through the two or more bonded wafers and the filter. The filter may be a woven mesh screen or preferably an electroformed screen with predetermined pore size. Since the filter covers one entire side of the printhead, a relatively large contact area prevents delamination and enables convenient leak-free sealing.

One problem associated with thermal ink jet technology is the sensitivity of ink droplet directionality to particulates in the ink. Print quality is directly related to accurate placement of the ink droplets on a recording medium and droplet directionality determines the accuracy of the ink droplet placement. It has been demonstrated that higher print quality is achieved with particulate-free ink sources and the degree of particulate-free ink is related to how close the final filtration of the ink is to the ink jet printhead. One source of particulate contamination is the manufacturing environment itself. At least a partial solution to particulate-induced misdirectionality problems is to construct the entire transducer structure in a clean environment. However, complete particle-free environments are not practical. This invention solves the problems of particle contamination during the fabrication of an ink jet printhead.

It is object of the present invention to provide an ink jet printhead having an ink supply inlet covered by an integral membrane filter and method of fabrication therefor.

It is another object of the invention to provide an integral membrane filter over the inlet of the printhead by laminating a layer of photopatternable material over an etch-resistant mask layer on the bottom or reverse of a previously completed channel plate wafer and photoprocessing these layers to form the filter.

In the present invention, a plurality of ink jet printheads with integral membrane filters are fabricated from two (100) silicon wafers. A plurality of sets of heating elements and their individually addressing electrodes are formed on the surface of one of the wafers and a corresponding plurality of sets of parallel channel grooves, each channel groove set communicating with a recessed manifold, are formed in a surface of the other wafer. The two wafers are aligned and bonded together and individual printheads are obtained by a sectioning operation which cuts the mated wafers into a plurality of printheads. The integral membrane filter is formed on the channel wafer after it is anisotroically etched and prior to mating with the heater plate wafer. A photopatternable layer is deposited over the etch resistant masking layer and exposed, patterned and developed to establish the mesh filter.

In another embodiment, a side of the channel wafer not patterned and etched is heavily doped to increase the robustness of the membrane filter by functioning as an etch stop. This doped region beneath the patternable layer is then etched using the membrane filter as a mask to open the filter pores through the doped layer portion of the channel wafer.

In addition to filtering contamination from the ink and ink supply system during printing, the membrane filter also keeps dirt and other contamination from entering the large ink inlets during printhead assembly. The foregoing features and objects will become apparent from a reading of the following specification in conjunction with the drawings, wherein like parts have the same index numerals.

FIG. 1 is a partially shown enlarged isometric view of a single printhead having the integral membrane filter of the present invention and showing the ink droplet emitting nozzles.

FIG. 2 is a cross-sectional view of the printhead as viewed along view line 2--2 of FIG. 1.

FIG. 3A-3F show partial cross-sectional views of the channel plate fabricating steps which include the fabrication of the integral membrane filter.

FIGS. 4A-4E show partial cross-sectional views of the channel plate fabricating steps of an alternate embodiment of the present invention.

In FIGS. 1 and 2, a thermal ink jet printhead 10 of the present invention is shown comprising channel plate 12 with integral membrane filter 14 and heater plate 16 shown in dashed line. A patterned film layer 18 is shown in dashed line having a material such as, for example, Riston®, Vacrel®, or polyimide, and is sandwiched between the channel plate and the heater plate. As disclosed in U.S. Pat. No. 4,774,530 to Hawkins and incorporated herein by reference in its entirety, the thick film layer is etched to remove material above each heating element 34, thus placing them in pits 26, and to remove material between the closed ends 21 of ink channels 20 and the manifold 24 forming trench 38 in order to place the channels into fluid communication with the manifold. For illustration purposes, droplets 13 are shown following trajectories 15 after ejection from the nozzles 27 in front face 29 of the printhead.

Referring to FIG. 1, the printhead comprises a channel plate 12 that is permanently bonded to heater plate 16 or to the patterned thick film layer 18 optionally deposited over the heating elements and addressing electrodes on the top surface 19 of the heater plate and patterned as taught in the above-mentioned U.S. Pat. No. 4,774,530. The channel plate is silicon and the heater plate may be any insulative or semiconductive material as disclosed in the U.S. Pat. No. Re. 32,572 to Hawkins et al. The present invention is described for an edge-shooter type printhead, but could readily be used for a roofshooter configured printhead (not shown) as disclosed in U.S. Pat. No. 4,864,329 to Kneezel et al, wherein the ink inlet is in the heater plate, so that the integral filter of the present invention could be fabricated in an identical manner thereover. The description of FIGS. 8 and 9 of U.S. Pat. No. 4,864,329 is herein incorporated by reference.

Channel plate 12 of FIG. 1 contains an etched recess 24, shown in dashed line, in one surface which, when mated to the heater plate 16, forms an ink reservoir or manifold. A plurality of identical parallel grooves 20, shown in dashed line and having triangular cross sections, are etched in the same surface of the channel plate with one of the ends thereof penetrating the front face 29 thereof. The other closed ends 21 (FIG. 2) of the grooves are adjacent the recess 24. When the channel plate and heater plate are mated, the groove penetrations through edge 29 produce the orifices or nozzles 27 and the grooves 20 serve as ink channels which connect the manifold with the nozzles. The open bottom 25 of the manifold in the channel plate, shown in FIG. 2, provides means for maintaining a supply of ink in the manifold from an ink supply source (not shown). Filter 14 of the present invention has been fabricated, as discussed later, by depositing a photopatternable layer 23 over the etch resistant mask layer 22 and photodelineating a pattern of apertures or pores 28 having predetermined sizes in the range of 5-30 μm in length, width, or diameter through the two layers in an area equal to and in alignment with the open bottom 25 of the manifold 24. The layers 22, 23 may cover the entire upper surface 17 of the channel plate or may be etch removed to an area slightly larger than the manifold open bottom which serves as the ink inlet, as shown in FIGS. 1 and 2. The filter size must be large enough to provide an adequate seal around the manifold open end 25 with enough adhering surface area to prevent delamination. Referring to FIG. 2, the distance "t" represents the minimum dimension between the outer periphery of the filter 14 and the internal dimensions of the open end 25; the distance t is preferably about 25 to 50 μm.

The fluid resistance is very low because the filter is extremely thin and can be made with relatively high fluid transmission values. For example, a 4 micron thick filter in a 1,000 line per inch square grid pattern with an 18 micron square pore size, which is suitable for the present invention, has a transmission value of 50%. Other pore shapes and filter thicknesses are acceptable, so long as the pore area is about 300 square microns and the filter thickness is within 3 to 10 μm.

In addition to filtering out contamination from the ink and ink supply system during printing, the filter also keeps dirt and other debris from entering the relatively large inlets during printhead assembly. In this way, it is possible to use less strigently clean and, therefore, less expensive assembly rooms for printhead manufacture, after the filter has been bonded in place. Operations up through assembly of the filter onto the bonded channel and heater wafers will need to occur in a clean room or under a clean hood, while subsequent operations can compromise somewhat on cleanliness.

The fabricating process for the channel plate having an integral membrane filter over its ink inlet is shown in FIGS. 3A-3F, each being partial, cross-sectional views of a (100) silicon wafer 12A and showing substantially only one of a plurality of channel plates 12. After the wafer is chemically cleaned, an etch resistant mask layer such as a pyrolytic CVD silicon nitride layer 22 is deposited to a thickness of about 1,000 Å on both top and bottom sides, 17 and 11 respectively. In FIG. 3A, the silicon nitride layer 22 on the bottom side 11 of the wafer (and channel plate) is photolithographically patterned to form a relatively large rectangular via 30 and a set of elongated, parallel vias 32. As disclosed in the above-mentioned reissue patent to Hawkins and shown in FIG. 3B, a potassium hydroxide (KOH) anisotropical etchant is used to etch the bottom surface 11 of the wafer 12A and form the channel grooves 20 and manifold recess 24. The manifold recess is etched through the wafer to provide an open bottom 25 that is closed by the etch resistant mask or silicon nitride layer 22 on the top surface 17 of the wafer.

It can be seen from FIG. 3C that the silicon nitride layer 22 covering the open bottom 25 of the manifold recess 24 could be photolithographically patterned and plasma etched to produce pores 28 and thus provide an integral filter 14A. However, this pyrolytic nitride membrane filter is very thin and highly stressed, so that is tends to be too fragile for use in a commercial ink jet printer.

To ensure the strength of the membrane filter, the preferred embodiment of the filter is produced by depositing a much thicker film of plasma enhance CVD nitride 23 over the pyrolytic silicon nitride layer 22, as shown in FIG. 3D. The plasma enhanced nitride layer is deposited to a thickness of about 15,000 Å, and, since it is more than an order of magnitude thicker than the pyrolytic silicon nitride layer, it can be deposited with very low or negligible stress, and therefore provides for a much stronger membrane from which the filter pores 28 can be photolithographically fabricated to produce the integral filter 14, as shown in FIGS. 3E and 3F. In FIG. 3E, the plasma enhanced silicon nitride layer 23 is shown photolithographically patterned to produce the pores 28 therein as well as reduce the size to one whose outer periphery that surrounds the pores 28 is 2t dimensionally larger than the manifold inlet (open bottom) 25. Using the patterned plasma enhance silicon nitride as a mask the membrane filter is completed as shown in FIG. 3F. Dashed line 29A represents the cutting lines for separating the printheads after alignment and bonding of heater plate and channel plate wafers together, so that the channels 20 are opened to form nozzles 27.

In one alternate embodiment (not shown), the plasma enhanced silicon nitride layer is replaced by laminating a layer of Vacrel® having a thickness of 2 to 4 μm on the pyrolytic silicon nitride layer on the top surface 17 of the wafer 12A, and then photoprocessing the two layers to form the integral membrane filter. Prior to laminating the Vacrel® layer on the pyrolytic silicon nitride layer, a thin coat of epoxy may be applied to improve its adhesion to the Vacrel®. As in the preferred embodiment, the filter pattern can be over the entire wafer surface or only over the ink inlet and surrounding edge which is large enough to provide a good seal and bonding area to prevent delamination. After curing, the channel wafer with integral filters is ready to be aligned and bonded to the heater wafer and then diced into individual printheads 10.

Another embodiment of the integral membrane filter and fabricating process therefor is shown in FIGS. 4A through 4E. To successfully apply the strengthening, substantially stress-free photopatternable layer 23, for example, plasma enhanced silicon nitride or polyimide, the pyrolytic silicon nitride layer must remain intact over the manifold open bottom (ink inlet). To ensure the highly stressed pyrolytic silicon nitride layer 22 remains intact and to add more strength and robustness to the membrane filter the top surface 17 of the silicon wafer 12A, the wafer surface is heavily doped with, for example, boron. The dopant may be applied by ion implantation or by using a spin on dopant and drive diffusion to form a doped layer 40 having a depth of about 2 to 10 μm, as shown in FIG. 4A in dashed line. In FIG. 4B, a pyrolytic silicon nitride layer 22 is deposited on both top and bottom surfaces 17, 11 of the wafer 12A, respectively, in the same manner as described above in FIG. 3A. The bottom silicon nitride layer is photolithographically patterned to produce a plurality of sets of parallel elongated vias 32 and associated manifold producing vias 30, followed by anisotropically etching the exposed wafer surfaces through the vias to produce the channel grooves 20 and manifold recess 24 as shown in FIG. 4C. The doped layer 40 of the silicon wafer acts as an etch stop and produces a manifold recess bottom 25A. The integral membrane filter is formed in FIGS. 4D and 4E by first depositing a patternable layer 23 such as plasma enhanced silicon nitride or polyimide and photolithographically forming pores 28, therein over and in alignment with the manifold bottom 25A, followed by etching of the prolytic silicon nitride layer 22, using the pores in layer 23 as a mask, and then plasma etching of the pores through the doped layer 40 in the silicon wafer 12A using the patterned pores in the patternable layer 23 and silicon nitride layer 22 as a mask. Dashed line 29A in FIG. 4E shows the dicing line to produce the printhead front face 29 and open the channel grooves to form the nozzles 27 after the channel wafer 12A is aligned and bonded to the heater wafer (not shown)

Many mofifications and variations are apparent from the foregoing description of the invention and all such modifications and variations are intended to be within the scope of the present invention.